Present day manufacturing for semiconductor electronics, solar cells, and other technology relies on ion implanter systems for doping or otherwise modifying silicon and other types of substrates. A typical ion implanter system performs the doping by generating an ion beam and steering it into a substrate so that the ions come to rest beneath the surface. Different types of ion implantation systems have been developed for different applications. High-current ion implanter systems are one type of implanter system that is widely used in semiconductor manufacturing. Such implanter systems typically produce currents up to 25 milliamperes (mA) and may be employed to efficiently introduce high doses of implanted species into a substrate.
Medium-current ion implanter systems have been developed to produce an ion beam having an intensity in the range of one microampere to about five mA, at energies between 2 kilo electron volts (keV) and 900 keV. These types of ion implantation systems may be especially useful to introduce dopant into a substrate in concentration ranges of about 1E13 to 5E14 or so. Generally, medium current implanter systems have been developed to operate by scanning a spot beam across a wafer In particular, for many applications, during ion implantation, it is desirable to achieve a uniform ion dose or beam current profile along the scan path. One approach to achieve this is to scan a spot beam in one plane while moving a target wafer in a direction orthogonal the plane to treat an entire surface of the target wafer. Scanning of an ion beam may be accomplished by the use of electrostatic scanners that are employed to controllably deflect the ion beam from its normal trajectory to span a larger area by changing the electric fields in a direction perpendicular to the direction of travel of the ion beam. The strength of the scanner field determines the total deflection from the normal path of the ion beam, hence the ion beam may be scanned by changing the electric field strength of the scanner elements.
FIG. 1a depicts an ion implantation system 100 that is arranged according to the prior art. As illustrated, the ion implantation system 100 includes an ion source 102, which typically is used to generate positive ions for implantation. The positive ions are provided as an ion beam that is deflected, accelerated, decelerated, shaped, and/or scanned between its emergence from the ion source and a substrate to be processed. An ion beam 120 is illustrated in FIG. 1 by a central ray trajectory (CRT). However, it will be appreciated by those of skill in the art, that an ion beam has a finite width, height, and shape, which may vary along the beam path between the ion source 102 and substrate 112. FIG. 1a further depicts a mass analyzer 104 that deflects the ion beam, an electrostatic scanner 106, corrector magnet 108, and end station 110 that may manipulate the substrate 112. In known systems, the electrostatic scanner 106 generates an electric field that is generally perpendicular to the direction of travel of ion beam 120 as it passes through the electrostatic scanner 106.
FIG. 1b illustrates a known scenario in which a spot beam is used to implant a substrate. In the example shown, the substrate 112 is a circular wafer, such as a silicon wafer. FIG. 1b depicts a cross-section of the ion beam 120 projected onto the substrate 112. In known systems, it is typical for a scanner, such as the electrostatic scanner 106, to scan an ion beam along a direction, such as a direction 122 (shown as parallel to the X-axis of the Cartesian coordinate system illustrated), while the substrate 112 is independently translated along a second direction 124 (shown as parallel to the Y-axis), which may be perpendicular to the first direction. The action of translating the substrate along direction 124 together with the scanning of ion beam 120 along the direction 122 may allow the ion beam 120 to expose the entire substrate 112 to ions. In the example illustrated, the ion beam 120 is a spot beam having a height H1 and width W1.
As shown in FIG. 1b, when the ion beam 120 is scanned along the direction 122 the ion beam 120 covers a scanned area 126. Because of the size and shape of the ion beam 120 and shape of the substrate 112, in order to ensure that all desired regions of the substrate 112 are exposed to the ion beam 120, the ion beam 120 is typically scanned beyond the edge 128 of the substrate 112 as illustrated. For example, it may be necessary to scan the ion beam 120 past the edge 128 a distance comparable to or even larger than width W1, as suggested in FIG. 1b. The scanned area 126 may thus include a substantial region 130 (shown only along one side of the substrate 112 for clarity) that lies outside of the substrate 112 and represents a dose of ions that is “wasted,” that is, the ions in region 130 are not used to implant or otherwise treat the substrate 112.
In addition, if the height H1 of the ion beam 120 is not sufficiently large, implantation dose non-uniformities may result. It may be desirable to ensure that the height H1 is not so large that ions strike beamline components such as pole pieces of corrector magnets that may be arranged to surround the ion beam 120. However, if the value of H1 is too small, the substrate 112 may be non-uniformly implanted when the substrate 112 is translated along the direction 124. For example, an ion beam 120 may oscillate in the direction 122 when the substrate is located at position P1, leading to implantation in an area on the substrate 112 that corresponds to the portion of the scanned area 126 that impinges on the substrate 112. The substrate 112 may then be stepped or scanned along the direction 124, leading to successive areas of comparable size to scanned area 126 being exposed on the substrate 112 due to the action of the electrostatic scanner 106. However, due to the finite dimension for the ion beam 120 along the direction 124, that is height H1, there may be underlap or overlap of the successive areas exposed by the scanning of ion beam 120 along the direction 122.
In order to improve uniformity in such ion implantation systems, it may be desirable to alter the beam size and or shape of an ion beam in cross-section. For example, extra lens elements may be added to the beamline to alter the beam shape, such as a lens to increase the beam spot size. However, the introduction of extra lens elements serves to increase the ion beam path length and to change the footprint of an ion implantation system, both of which are generally not desirable. In addition, the introduction of electrostatic scanners in series with components such as lens elements to shape the ion beam may create an increased region in which electrons are stripped from the ion beam. As is known, whenever electron are stripped or removed from a (positive) ion beam, the ion beam has a tendency to expand. This takes place due to the mutual repulsion of positive ions within the ion beam. The ion beam may be stripped of electrons anytime low energy electrons are attracted and accelerated out of the ion beam by a high positive potential applied to any of various beamline components. A result of beam expansion may include a reduction in beam current that can effectively be applied to a substrate.
What is needed is an improved method and apparatus to form more uniform beams in ion implantation systems, such as medium current ion implantation systems.